SEQUENCE STRATIGRAPHY AND FACIES ANALYSIS OF THE ROLLINS SANDSTONE MEMBER (MOUNT GARFIELD FORMATION) AND RE-EXAMINATION OF THE CONTACT BETWEEN THE MOUNT GARFIELD AND WILLIAMS FORK FORMATIONS (LATE CRETACEOUS)

Abstract

The Cretaceous Rollins Sandstone Member (Mount Garfield Formation) is the youngest marine sandstone deposited within the Sevier foreland basin in Colorado. The Rollins Sandstone Member is a complicated stratigraphic unit that consists of four progradationally stacked sequences. These sequences were deposited as a result of high-frequency changes in sea level. Each sequence initiates with an incised valley fill and contains a single parasequence within the highstand systems tract. Parasequences within highstand systems tracts contain offshore to marine-shoreface deposits. The top of the Rollins Sandstone Member is a surface that results from the progradation of a single strandline. This surface can be used as a regional datum. This new datum indicates there is no upward-climbing geometry at the top of the Mount Garfield Formation, and the Rollins Sandstone Member and the Cameo Wheeler coal zone (of the Williams Fork Formation) are not time-equivalent units. The marine- shoreface deposits within the Rollins Sandstone Member represent high-energy shorefaces. These shorefaces had daily wave heights of 1-2 m and Nor'easter-scale storms occurring several times a year. These high-energy conditions produced a straight coastline along the western edge of the Cretaceous Western Interior seaway.